In conjunction with demand for improved fuel economy for automobiles in recent years, there has been demand for conjugated diolefin based rubber as a rubber material for tires that provides low rolling resistance, wear resistance, excellent fracture properties, as well as steering stability. Previously known technologies include Japanese Unexamined Patent Application H12-178378. This technology increases compatibility between the conjugated diene-based polymer and silica, and increases silica dispersibility, by adding a modifying group that interacts with silica on the end of the polymer, specifically an amino group. However, this effect is not always sufficient and there is desire for further improvement.